Matplotlib: Plotting multiple histograms in plt.subplots

Question

Background of the problem:

I'm working on a class that takes an Axes object as constructor parameter and produces a (m,n) dimension figure with a histogram in each cell, kind of like the figure below:

There are two important things to note here, that I'm not allowed to modified in any way:

The Figure object is not passed as a constructor parameter; only the Axes object is. So the subplots object cannot be modified in any way.
The Axes parameter is set to that of a (1,1) figure, by default (as below). All the modification required to make it an (m,n) figure are performed within the class (inside its methods)

_, ax = plt.subplots() # By default takes (1,1) dimension
cm = ClassName(model, ax=ax, histogram=True) # calling my class

What I'm stuck on:

Since I want to plot histograms within each cell, I decided to approach it by looping over each cell and creating a histogram for each.

results[col].hist(ax=self.ax[y,x], bins=bins)

However, I'm not able to specify the axes of the histogram in any way. This is because the Axes parameter passed is of default dimension (1,1) and hence not index-able. When I try this I get a TypeError saying.

TypeError: 'AxesSubplot' object is not subscriptable

With all this considered, I would like to know of any possible ways I can add my histogram to the parent Axes object. Thanks for taking a look.

if you only get the ax reference like that I don't think there's much you can do since you will be stuck with a 1-row, 1-column matrix on where to plot (hence a single plot). — Luca Giorgi
– Luca Giorgi, Commented Feb 18, 2019 at 15:59
If I'm given the figure object too, is there anything I can do? — Ajay Subramanian
– Ajay Subramanian, Commented Feb 18, 2019 at 16:01
I don't think so but I'm not super experienced with matplotlib. To me it looks like there's either an error in how the figure is instanced if you are not allowed to touch it or otherwise you can modify it / create a new one and hence you can just use plt.subplots(2,2) — Luca Giorgi
– Luca Giorgi, Commented Feb 18, 2019 at 16:05

ImportanceOfBeingErnest · Accepted Answer · 2019-02-18 18:23:33Z

The requirement is pretty strict and maybe not the best design choice. Because you later want to plot several subplots at the position of a single subplot, this single subplot is only created for the sole purpose of dying and being replaced a few moments later.

So what you can do is obtain the position of the axes you pass in and create a new gridspec at that position. Then remove the original axes and create a new set of axes at within that newly created gridspec.

The following would be an example. Note that it currently requires that the axes to be passed in is a Subplot (as opposed to any axes). It also hardcodes the number of plots to be 2*2. In the real use case you would probably derive that number from the model you pass in.

import matplotlib.pyplot as plt
import numpy as np
from matplotlib import gridspec

class ClassName():
    def __init__(self, model, ax=None, **kwargs):
        ax = ax or plt.gca()
        if not hasattr(ax, "get_gridspec"):
            raise ValueError("Axes needs to be a subplot")
        parentgs = ax.get_gridspec()
        q = ax.get_geometry()[-1]

        # Geometry of subplots
        m, n = 2, 2
        gs = gridspec.GridSpecFromSubplotSpec(m,n, subplot_spec=parentgs[q-1])
        fig = ax.figure
        ax.remove()

        self.axes = np.empty((m,n), dtype=object)
        for i in range(m):
            for j in range(n):
                self.axes[i,j] = fig.add_subplot(gs[i,j], label=f"{i}{j}")


    def plot(self, data):
        for ax,d in zip(self.axes.flat, data):
            ax.plot(d)


_, (ax,ax2) = plt.subplots(ncols=2) 
cm = ClassName("mymodel", ax=ax2) # calling my class
cm.plot(np.random.rand(4,10))

plt.show()

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